Force TreeDropdown to remember selected title.

`.TreeDropdownField.searchable::resetTitle()` is called (line 285) when the user clicks away from the dropdown field, and this function uses `data('title')` to reset the title. However this is never set when the user selects something! So it always displays the last title - "(Choose)" - after a selection.

This change sets `data('title')` to the title of the chosen node when it is selected.